System and method for storage and retrieval of energy

ABSTRACT

A system and method for storage and retrieval of energy includes storage of energy by performing electrolysis of water and directing the resulting hydrogen gas and oxygen gas to respective storage tanks. Energy is retrieved by directing hydrogen gas and oxygen gas from the storage tanks to a fuel cell where the hydrogen and oxygen are reacted to produce electricity and water. Water from the fuel cell is directed to a reservoir for subsequent electrolysis.

RELATED APPLICATIONS

Not applicable.

BACKGROUND AND FIELD 1. Field

The present device relates generally to methods for energy storage, andmore specifically to a method for storage and retrieval of energyutilizing electrolysis and fuel cell technology.

2. Background

As new technologies are developed, particularly in the area of renewableenergy and renewable energy-powered devices, new methods of storing andretrieving energy are needed. Advances in batteries, and specificallywith respect to lithium ion batteries, have been promising, howeverdrawbacks remain. Intercalation causes degradation of lithium ionbatteries during charge/discharge cycles. Further, the chemistry of alithium ion battery places inherent limits on the energy density thatcan be achieved despite advances in technology. Replacements for lithiumhave been utilized, and research continues into improvements in lithiumion and related batteries. New and efficient methods of energy storageand retrieval are nonetheless needed.

SUMMARY

The present disclosure provides a method for storage and retrieval ofenergy. Energy is stored by performing electrolysis of water anddirecting the resulting hydrogen gas and oxygen gas to respectivestorage tanks. Energy is retrieved by directing hydrogen gas and oxygengas from the storage tanks to a fuel cell where the hydrogen and oxygenare reacted to produce electricity and water. Water from the fuel cellis directed to a reservoir for subsequent electrolysis.

In one aspect of the present method, the water electrolyzed to producehydrogen gas and oxygen gas may be provided at high pressure.

In another aspect of the present method, a pump may draw water from thereservoir and force the water into an electrolysis vessel at highpressure.

Another aspect of the present disclosure provides a system for storageand retrieval of energy. The system includes a water reservoir, anelectrolyzer in fluid communication with the water reservoir, and a pumpoperable to draw water from the water reservoir and direct the waterinto the electrolyzer. A first storage vessel is provided in fluidcommunication with the electrolyzer, and receives and stores oxygen gastherefrom. A second storage vessel is provided in fluid communicationwith the electrolyzer and receives and stored hydrogen gas therefrom. Afuel cell is in fluid communication with both the first and secondstorage vessels.

In another aspect of the present disclosure, the fuel cell of the systemis in fluid communication with the water reservoir, and water producedby the fuel cell is directed into the water reservoir.

In another aspect of the present disclosure, the pump of the system isconfigured to direct water into the electrolyzer at high pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic of a cyclical embodiment of a system of thepresent disclosure.

DETAILED DESCRIPTION

The present system allows for storage and retrieval of energy viaefficient conversion between water and its components elements, hydrogenand oxygen. The system disclosed herein is capable of operation over awide range of operating conditions and is preferably able to store afull day's worth of excess electricity production capacity for any givenuse to which an implementation is dedicated.

The present system and method include an energy storage step orcomponent, and an energy retrieval step or component. The energyretrieval step or component may also be referred to herein as an energyproduction step or component.

Energy storage is performed by electrolysis of water within aelectrolysis vessel. It is preferred that the water be compressed priorto electrolysis for efficiency purposes. The process of electrolysiscauses the water to decompose into hydrogen gas and oxygen gas. Thehydrogen gas resulting from the electrolysis flows along a conduit to astorage vessel, where the hydrogen gas is retained until it is requiredfor the energy retrieval step or component of the present system ormethod. Oxygen gas is likewise directed along a conduit to a separatestorage vessel, where it is also stored until needed for energyretrieval.

Energy retrieval or production is performed by directing hydrogen gasand oxygen gas from their respective storage vessels into a fuel cell.The hydrogen and oxygen gases are reacted within the fuel cell toproduce electricity and water. It is preferred that the water isdirected back to a reservoir that initially provides the water forelectrolysis, and that the water is stored there for use in a subsequentelectrolysis step.

A hypothetical system according to the present disclosure is nowdescribed. For purposes of this hypothetical, it is assumed that thesystem must meet an energy storage requirement of 118 kWh/day.

Net energy retrieved by the present system may be represented by thefollowing expression:(Energy In−Pumping Energy)*η_(E)*η_(FC)where η_(E)=electrolyzer efficiency and η_(FC)=fuel cell efficiency. Thesystem efficiency can therefore be expressed as the ratio of energyretrieved to energy in, as follows:

$\eta_{System} = {( {1 - \frac{E_{p}}{E_{i\; n}}} )*\eta_{E}*\eta_{FC}}$where E_(p)=pumping energy and E_(in)=energy in.

Storage pressure of water within the system is preferably 500 psi(3.45e6 Pa). The pumping power required to raise water pressure withinthe system from atmospheric pressure to storage pressure at 25° C. is3356 J/kg water. Assuming 32.3 kg of water are electrolyzed on a dailybasis, the daily energy requirement for pumping is equal to 108.3 kJ.The daily energy required for electrolysis of 32.3 kg of water is equalto 425,000 kJ. The pumping power therefore represents approximately0.025% of the total energy requirement of the system. This is negligiblefor purposes of this hypothetical and will be disregarded.

Given an average mass flow rate of 32.3 kg per 14 hours (6.41e-4 kg/s),the power required to pressurize the water is equal to 3.356kJ/kg*9.13e-4 kg/s, which is equal to 2.06 W or 0.003 hp.

Commercially available electrolyzers range in efficiency from at least80% up to around 95%. Fuel cell efficiency ranges from at least 50% upto around 85%.

Given that the pumping energy required by the present system is small,as noted above, the expression for system efficiency can be simplifiedto:η_(System)=η_(E)*η_(FC)Given the upper limits for fuel cell and electrolyzer efficiencies,above, it is assumed for purposes of this hypothetical that electrolyzerefficiency is 95% and fuel cell efficiency is 85%. Thus the overallsystem efficiency, η_(System), is 81%. It is contemplated, however, thathigher efficiencies may be possible through design optimization.

Temperature and pressure may also affect the efficiency of the presentsystem. As a general matter, efficiency of an electrolyzer decreaseswith increasing pressure and increases with increasing temperature. Withrespect to fuel cells, increases in temperature tend, generally, toresult in decreasing efficiency.

Achieving the expected energy storage and output requires mass storagefor use in the present system. Given that the energy required toelectrolyze water on a daily basis is 237.13 kJ/mole=13,159.3 kJ/kg=3.66kWh/kg, and an estimated daily energy input to be stored of 118 kWh/day,the mass of water required to store the daily energy input is

${32.28\mspace{14mu}{kg}\text{/}{day}} = {\frac{118\mspace{14mu}{kWh}\text{/}{day}}{3.66\mspace{14mu}{kWh}\text{/}{kg}}.}$The daily mass of hydrogen to be stored is

${3.62\mspace{14mu}{kg}\text{-}H_{2}\text{/}{day}} = {{32.28\mspace{14mu}{kg}} - {\frac{water}{day}*{\frac{2.018\mspace{14mu}{kg}\text{-}H_{2}}{18.02\mspace{14mu}{kg}\text{-}{water}}.}}}$The daily mass of oxygen to be stored is

${28.67\mspace{14mu}{kg}\text{-}O_{2}\text{/}{day}} = {{32.28\mspace{14mu}{kg}} - {\frac{water}{day}*{\frac{16.0\mspace{14mu}{kg}\text{-}O_{2}}{18.02\mspace{14mu}{kg}\text{-}{water}}.}}}$

Volume storage requirements must also be determined. Assuming the idealgas equation of state, wherein pv=RT→v=RT/p, where v=gas specific volume(m³/kg), R=the gas constant for each constituent (4.10 kJ/kg-K forhydrogen and 0.260 kJ/kg-K for oxygen), T=298 K, and p=3450 kPa (500psi), the storage volume of gases is V=mv=0.0013 m³ for hydrogen andV=mv=0.0006 m³ for oxygen. These volumes are accommodated bycommercially-available high-pressure cylinders.

Given the above parameters of the hypothetical, overall systemefficiency is estimated at approximately 80%. Storage requirements aremodest, and commercially-available storage cylinders provide a low-costmeans for storing the hydrogen and oxygen gas.

Turning now to the drawing, wherein like numerals indicate like parts,FIG. 1 provides a schematic of one, cyclical embodiment of a system 10according to the present disclosure. Although the system is cyclical,for purposes of describing this exemplary embodiment, this text willbegin with low-pressure reservoir 12, in which water is stored prior toelectrolysis. A pump 14 is provided in fluid communication withlow-pressure reservoir 12 and operable to draw water from low-pressurereservoir 12 along a first conduit 16, and to force the water along asecond conduit 18 into an electrolysis vessel 20 (also referred toherein as an electrolyzer). Electrolyzer 20 utilizes power to causedecomposition of the water therein into its constituents—hydrogen gasand oxygen gas. Oxygen gas is directed from the electrolyzer 20 along aconduit 22 into a first storage vessel 24. Hydrogen gas is directed fromelectrolyzer 20 along a conduit 26 into a second storage vessel 28. Thestorage vessels may be compressed gas cylinders, although any suitablestorage structures may be utilized.

When energy production is required, oxygen gas is directed from firststorage vessel 24 along conduit 30 into fuel cell 34. Likewise, hydrogengas is directed from second storage vessel 28 along a conduit 32 intofuel cell 34. The oxygen gas and hydrogen gas are reacted within thefuel cell to produce electricity and water. The electricity is directedaway from the fuel cell and used to perform work or, potentially, to bedirected to an electrical grid. The water produced by fuel cell 34 ispreferably directed along conduit 36 to low-pressure reservoir 12.

The various features and embodiments of the present device disclosedabove are illustrative of the present disclosure and are meant to beexemplary. Various modifications or alterations to what is disclosedherein may be readily apparent to those of skill in the art upon readingthis disclosure, and it is contemplated that such modifications oralterations remain within the spirit, and scope, of the presentinvention.

Having thus described the preferred embodiment of the invention, what isclaimed as new and desired to be protected by Letters Patent includesthe following:
 1. A method for storage and retrieval of energycomprising the steps of: (a) storing energy by: (i) performingelectrolysis of water within a electrolysis vessel such that the waterdecomposes into hydrogen gas and oxygen gas; (ii) directing the hydrogengas through a first conduit and into a first storage vessel for storagetherein; and (iii) directing the oxygen gas through a second conduit andinto a second storage vessel for storage therein; (b) retrieving energyby: (i) directing said hydrogen gas from said first storage vessel alonga third conduit into a fuel cell; (ii) directing said oxygen gas fromsaid second storage vessel along a fourth conduit into said fuel cell;and (iii) reacting the hydrogen gas and oxygen gas within the fuel cellto produce electricity and water, and (c) directing water from said fuelcell to a reservoir for subsequent electrolysis as in step a), above,wherein the electrolysis vessel, first storage vessel, second storagevessel, fuel cell, and reservoir form a closed loop.
 2. The methodaccording to claim 1, wherein the water electrolyzed in step (a)(i) isprovided at high pressure.
 3. The method according to claim 1, whereinthe water electrolyzed in step (a)(i) is directed from said reservoir tosaid electrolysis vessel along a fifth conduit.
 4. The method accordingto claim 3, wherein a pump associated with said fifth conduit drawswater from said reservoir and forces the water into said electrolysisvessel at high pressure.
 5. A system for storage and retrieval of energycomprising: a water reservoir; an electrolyzer in fluid communicationwith said water reservoir; a pump operable to draw water from said waterreservoir and to direct said water into said electrolyzer; a firststorage vessel in fluid communication with said electrolyzer andconfigured to receive oxygen gas therefrom; a second storage vessel influid communication with said electrolyzer and configured to receivehydrogen gas therefrom; a fuel cell in fluid communication with saidfirst storage vessel and said second storage vessel, wherein the waterreservoir, electrolyzer, first storage vessel, second storage vessel,and fuel cell form a closed loop.
 6. The system according to claim 5,wherein said fuel cell is in fluid communication with said waterreservoir, and further wherein water produced by said fuel cell duringoperation thereof is directed to said water reservoir.
 7. The systemaccording to claim 5, wherein said pump is configured to direct waterinto said electrolyzer at high pressure.